FIG. 1 is a schematic diagram showing general color matching between different devices.
In FIG. 1, input image data (RGB (red, green and blue) data or CMYK (cyan, magenta, yellow and black) data) represented in a device dependent color space is converted into XYZ data represented in a device independent color space, by using an input profile 11. Since the color outside the color gamut of an output device cannot be reproduced by the relevant output device, gamut mapping is executed to the XYZ data so that all the colors can be held within the color gamut of the relevant output device. Then, the XYZ data subjected to the gamut mapping is converted into the CMYK data represented in the device dependent color space (that is, the color space dependent on the output device). Here, numeral 12 denotes an output profile.
In the general color matching, a reference white point and an environment light are fixed. For example, in the profile defined by the ICC (International Color Consortium), the PCS (Profile Connection Space) for connecting the profile is defined as the XYZ value and the Lab value based on D50.
When an identical sample (for example, an image) is observed under different light sources, the XYZ values for the observed sample are of course different with respect to the respective light sources. For this reason, to estimate the XYZ values under the different light sources, various conversion (transformation) methods such as (1) ratio conversion, (2) von Kries transformation, (3) a prediction equation based on a color appearance model, and the like are proposed.
The ratio conversion is the method of executing the ratio conversion of W2/W1 to convert the XYZ value under the standard white point W1 into the XYZ value under the standard white point W2. If this method is applied to a Lab uniform color space, the Lab value under the standard white point W1 matches with the Lab value under the standard white point W2. For example, if it is assumed that the XYZ value of the sample under the standard white point W1 (Xw1, Yw1, Zw1) is (X1, Y1, Z1) and the XYZ value of the sample under the standard white point W2 (Xw2, Yw2, Zw2) is (X2, Y2, Z2), the following relation (1) is given based on the ratio conversion.X2=(Xw2/Xw1)·X1Y2=(Yw2/Yw1)·Y1Z2=(Zw2/Zw1)·Z1  (1)
The von Kries transformation is the method of executing the ratio conversion of W2′/W1′ on a human's color appearance space PQR to convert the XYZ value under the standard white point W1 into the XYZ value under the standard white point W2. If this method is applied to the Lab uniform color space, the Lab value under the standard white point W1 does not match with the Lab value under the standard white point W2. For example, if it is assumed that the XYZ value of the sample under the standard white point W1 (Xw1, Yw1, Zw1) is (X1, Y1, Z1) and the XYZ value of the sample under the standard white point W2 (Xw2, Yw2, Zw2) is (X2, Y2, Z2), the following relation (2) is given based on the von Kries transformation.
                                          [                                                                                X                    ⁢                                                                                  ⁢                    2                                                                                                                    Y                    ⁢                                                                                  ⁢                    2                                                                                                                    Z                    ⁢                                                                                  ⁢                    2                                                                        ]                    =                                                                      [                                      M                                          -                      1                                                        ]                                ⁡                                  [                                                                                                                                          P                            2                                                    /                                                      P                            1                                                                                                                      0                                                                    0                                                                                                            0                                                                                                                          Q                            2                                                    /                                                      Q                            1                                                                                                                      0                                                                                                            0                                                                    0                                                                                                                          R                            2                                                    /                                                      R                            1                                                                                                                                ]                                            ⁡                              [                M                ]                                      ⁡                          [                                                                                          X                      ⁢                                                                                          ⁢                      1                                                                                                                                  Y                      ⁢                                                                                          ⁢                      1                                                                                                                                  Z                      ⁢                                                                                          ⁢                      1                                                                                  ]                                      ⁢                                  ⁢                  where          ,                                          ⁢                                    [                                                                                          P                      1                                                                                                                                  Q                      1                                                                                                                                  R                      1                                                                                  ]                        =                                                                                [                    M                    ]                                    ⁡                                      [                                                                                                                        X                                                          w                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                                                                        Y                                                          w                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                                                                        Z                                                          w                              ⁢                                                                                                                          ⁢                              1                                                                                                                                            ]                                                  ⁢                                                                  [                                                                                                    P                        2                                                                                                                                                Q                        2                                                                                                                                                R                        2                                                                                            ]                            =                                                                                          [                      M                      ]                                        ⁡                                          [                                                                                                                                  X                                                              w                                ⁢                                                                                                                                  ⁢                                2                                                                                                                                                                                                                        Y                                                              w                                ⁢                                                                                                                                  ⁢                                2                                                                                                                                                                                                                        Z                                                              w                                ⁢                                                                                                                                  ⁢                                2                                                                                                                                                        ]                                                        ⁢                                                                          [                  M                  ]                                =                                                                            [                                                                                                    0.40024                                                                                0.70760                                                                                                              -                              0.08081                                                                                                                                                                                          -                              0.22630                                                                                                            1.16532                                                                                0.04570                                                                                                                                0                                                                                0                                                                                0.91822                                                                                              ]                                        ⁢                                                                                  [                                          M                                              -                        1                                                              ]                                    =                                      [                                                                                            1.85995                                                                                                      -                            1.12939                                                                                                    0.21990                                                                                                                      0.36119                                                                          0.63881                                                                          0                                                                                                                      0                                                                          0                                                                          1.08906                                                                                      ]                                                                                                          (        2        )            
The prediction equation based on the color appearance model is equivalent to the method of converting the XYZ value under the environment condition VC1 (including the standard white point W1) into the XYZ value under the environment condition VC2 (including the standard white point W2) by using a human's color appearance space QMh (or JCh) such as CIE CAM97s (Commission Internationale de l'Eclairage Color Appearance Model, 1997, Simple) or the like. Here, with respect to the human's color appearance space QMh, the symbol “Q” indicates “brightness”, the symbol “M” indicates “colourfulness” and symbol “h” indicates “hue quadrature” or “hue angle”. Further, with respect to the human's color appearance space JCh, the symbol “J” indicates “lightness”, the symbol “C” indicates “chroma” and the symbol “h” indicates “hue quadrature” or “hue angle”. In any case, if this conversion is applied to the Lab uniform color space, as well as the von Kries transformation, the Lab value under the standard white point W1 does not match with the Lab value under the standard white point W2. For example, if it is assumed that the XYZ value of the sample under the standard white point W1 (Xw1, Yw1, Zw1) is (X1, Y1, Z1) and the XYZ value of the sample under the standard white point W2 (Xw2, Yw2, Zw2) is (X2, Y2, Z2), the following conversion is executed according to the color appearance model.(X1, Y1, Z1)→[CIE CAM97s forward conversion]→(Q, M, H) or (J, C, H)→[CIE CAM97s inverse conversion]→(X2, Y2, Z2)  (3)
FIG. 2 is a conceptual diagram showing the color matching under different observation environments, to be executed by using the color appearance model.
In FIG. 2, the input data (RGB data, CMYK data, gray data, or the like) represented in the device dependent color space is converted into XYZ50 data represented in the device independent color space dependent on the input observation condition, by using an input profile 21 dependent on the input observation condition (viewing condition 1 D50). Then, a forward conversion process 22 is executed to the color appearance model based on the input observation condition (viewing condition 1) to convert the XYZ50 data into JCh data 23 represented in the color appearance space JCh or QMh data 24 represented in the color appearance space QMh. Subsequently, an inverse conversion process 25 is executed to the color appearance model based on the output observation condition (viewing condition 2) to convert the JCh data 23 or the QMh data 24 into XYZ65 data. Further, the acquired XYZ65 data is converted into the CMYK data represented in the color space dependent on the output observation condition and the device by using an output profile 26 dependent on the output observation condition (viewing condition 2).
As shown in FIGS. 1 and 2, the color matching between the different devices can be achieved through the device independent XYZ value (or Lab value).
However, it is necessary to consider the following problems in case of converting the XYZ value to the CMYK value. That is, (1) since there are the plural combinations of the CMY value and the K value in regard to one XYZ value, it is necessary to fix the characteristic of black printer (K printer) generation to acquire one solution. Further, (2) since there is a case where the total area coverage in the ink or the toner is not controlled in regard to a CMYK patch print-output from a CMYK device, it is necessary to set the total area coverage according to each medium and print quality in case of converting the XYZ value to the CMYK value.
Here, it is assumed that, in the total area coverage, the outputtable upper limit value is set in the combination of the respective inks and toners. For example, the area coverage of primary color (K), the area coverage of secondary color (C+M, M+Y, Y+C), and the area coverage of quartic color (C+M+Y+K) are set. Incidentally, it should be noted that the primary color is the color reproduced by using one kind of colorant, and the secondary color is the color reproduced by using two kinds of colorants. That is, an N-degree color is the color represented by using N kinds of colorants.
In the profile (that is, the XYZ value under the observing condition in each device is used instead of the PCS D50) which is used in consideration of the conventional ICC profile and the observation condition, the conversion table such as a 3D LUT (three-dimensional lookup table) or the like stored in case of converting the XYZ value to the CMYK value is used, whereby the black printer generation characteristic and the total area coverage of toner or ink have to be set at the time of generating the profile. Thus, it is necessary to generate the profile, install the generated profile, designate the installed profile and execute the color matching every time the setting of the black printer generation characteristic and the total area coverage is changed.
Moreover, in the profile which is used in consideration of the conventional ICC profile and the observation condition, the gamut mapping is included in the profile conversion. For this reason, when the profile is generated, the characteristic of the other profile becomes undefined. More specifically, when the output profile is generated, the characteristic of the input profile becomes undefined, and vice versa. Thus, it is impossible to execute the gamut mapping by using both the input-side color gamut and the output-side color gamut. In other words, it is impossible to execute the gamut mapping which is most appropriate for the combination of the input-side color gamut and the output-side color gamut (that is, the target of the color matching).